Jen-You Chu

Optimizing electromagnetic enhancement of flexible nano-imprinted hexagonally patterned surface-enhanced Raman scattering substrates

The production of inexpensive, large-scale, uniform substrates for surface-enhanced Raman scattering (SERS) is a key to popularize its usage in chemical and biological detection. We demonstrate a flexible nano-imprinted hexagonally patterned SERS-active substrate. Its electromagnetic enhancement factor was optimized by the thickness adjustment of its silver over-coated film. The experimental data show a good correspondence with the theoretical prediction. Such substrate was shown to exhibit high uniformity and reproducibility with a variation of less than 2%, offering a potential of greatly exploiting such substrate in infield biocide monitoring.

Effects of the tip shape on the localized field enhancement and far field radiation pattern of the plasmonic inverted pyramidal nanostructures with the tips for surface-enhanced Raman scattering

The plasmonic 2D W-shape and 3D inverted pyramidal nanostructures with and without the tips are studied. The effects of the tip height and tip tilt angle on the near field enhancement and far field radiation pattern are discussed in this paper. The localized hot spots are found around the pits and the radiation pattern can be affected by the tip structures. The inverted pyramidal nanostructures with and without the tips were fabricated and their reflection spectra and surface-enhanced Raman scattering (SERS) signals for the chemical molecules thiophenol were measured. The simulation according to the geometry parameters of the fabricated structures is demonstrated. We found that the SERS of our proposed structures with the tips can have stronger light field enhancements than the inverted pyramidal nanostructures without the tips, and the far field radiation pattern can be varied by changing the tip height and tip tilt angle. The study of surface plasmon modes and charge distributions can help the understanding of how to arrange the plasmonic structures to achieve high field enhancement and preferred far field radiation pattern. Our study can be useful for the design of the strong field enhancement SERS substrate with specific far field radiation properties. It can be also applied to the portable Raman detectors for in situ and remote measurements in specific applications.

Light trapping enhancements of inverted pyramidal structures with the tips for silicon solar cells

The anti-reflection properties of the inverted pyramidal structures with the tips for crystalline silicon solar cells are studied numerically and experimentally, and the comparisons with inverted pyramidal structures without the tips are given. Because the light has more multiple reflection between different surfaces of the inverted pyramidal structures with the tips and more light energies are trapped and absorbed by the solar cell materials, the structures with the tips can have smaller reflection. This study can be useful for the designs of textured structures for the solar cells or other anti-reflection applications.

Fourier analysis of surface plasmon waves launched from single nanohole and nanohole arrays: unraveling tip-induced effects

The authors report the investigation of surface plasmon waves (SPW) generated by single nanohole and nanohole arrays. Scattering-type scanning near-field microscopy is used to directly observe near-field distribution. The images after Fourier transformation display characteristic patterns that match with the derived analytic formula. The correspondence helps to identify the role of the scanning tip in generating SPW, making possible of the removal of this tip-induced effect. This study provides a means to perform in-depth investigation on surface plasmon polaritons.

Near field imaging of subwavelength plasmonic structures

This report presents an overview of our recent near-field investigation of both local and surface plasmon resonance (SPR) with a scattering-type scanning near-field optical microscope (s-SNOM) which has sub-10 nanometer resolution. With the ability to perform near-field optical experiments at multiple excitation wavelengths simultaneously, this instrument has recorded near-field intensity and phase images of a wide range of subwavelength plasmonic structures: single nanohole and nanoslit, circular and elliptical hole arrays, etc. The near-field results obtained with different excitation wavelengths were confirmed by numerical calculation and were made direct correspondence with far-field observations by comprehensive models. The multi-wavelength s-SNOM proves to be an essential tool to unravel many interesting plasmonic phenomena in nanometer scale. This work investigates the nature of subwavelength plasmon optics which potentially will play an important role in the development of many innovative highly efficient optoelectronic devices (light-emitting devices and solar cells) and highly sensitive sensors based on SPR and surfaceenhanced Raman scattering.

Light Trapping for Solar Cells

The textured microstructures and the photonic nanostructures for light trapping structures are discussed. The inverted pyramidal microstructures with and without the tips were fabricated, and their light trapping performances were measured by the reflectance spectroscopy and simulated by ray tracing method. It is found that there are more chances of light reflecting between the surfaces of the tips and the inverted pyramids. Four kinds of nanophotonic light trapping structures are simulated by the finite-difference time-domain method, and their optical transmissions and light trapping performances are calculated. The results show that mixing the dielectric and metallic nanoparticles or materials can have superior light trapping performances as comparing to using dielectric nanoparticles only or using metallic nanoparticles only. The other kinds of light trapping structures and their outlooks are also given.